In God of War, the hero God of War god Krotas must level up in order to keep pace with the increasingly difficult enemies he has to defeat. Also his son Atreus needs to level up too. In order to level up both Krotas and Atreus, the heroes need to level up the following, and make the gear (armor, armor parts, weapon, weapon parts)

Craft new armor, which includes the chest armor, the waist armor and the arms armor.

Craft new axe pommel.

Upgrade chest armor, upgrade waist armor and upgradep arm armor. In some epic and legendary armor, this will increase the number of enchantment slots. Put more enchantment into more slots to level up or make the hero more powerful.

This will cause the stats of the heroes to keep increasing. Stats include strength, vitality, runic luck and coooldown.

Later the hero will get reacquainted with his old flame of war. Keep upgrading this and unlock all skills.

As the story progresses, the hero will continue to level up. For example after the second escape from Helheim, the mist gear is unlocked. The mist gear allows for regeneration and also puts high stats into strength, runic, defence and vitality.

Also the hero will get runes. You can level up the rune with XP just like how you level up the skills.